In a catalyst system which reduces NOx discharged from a typical lean burn engine of a diesel engine by use of ammonia as a reducing agent to purify the exhaust gas, the use of an aqueous solution of urea which is a precursor of the ammonia (hereinafter referred to as “the urea aqueous solution”) as an ammonia generating source has been investigated (JP-A-2000-027627 or the like), and such a technology has already been put to practical use.
In the catalyst system where such an urea aqueous solution is used (hereinafter referred to as “the urea SCR”), a selective reduction type NOx catalyst (hereinafter referred to simply as “the NOx catalyst”) is disposed in an exhaust passage of the engine, and a reducing agent adding device which injects and supplies the urea aqueous solution to the exhaust gas is further disposed on an upstream side of this NOx catalyst. The urea in the urea aqueous solution supplied to the exhaust gas by the reducing agent adding device causes hydrolysis by exhaust heat, to generate the ammonia, and this ammonia reacts with NOx in the exhaust gas on a catalyst layer of the NOx catalyst, whereby the NOx is reduced to purify the exhaust gas.
Here, ammonia which passes through the NOx catalyst without reacting with the NOx (hereinafter, this phenomenon will be referred to as “the ammonia slip”) is present, depending on an operation state of the engine. The ammonia slip is stipulated in a technical guideline (“Technical Guideline of Urea Selective Reduction Type Catalyst System”) established by Road Transport Bureau of the Ministry of Land, Infrastructure, Transport and Tourism, and the above stipulation is important for the practical use of the urea SCR not only to decrease an amount of the NOx to be discharged into the atmosphere but also to suppress the ammonia slip as much as possible.
In a generally employed NOx catalyst such as a zeolite catalyst, an adsorption ability thereof is utilized to adsorb ammonia on the catalyst layer at a low temperature, whereby opportunities for contact between ammonia and NOx are increased, and an NOx purification ratio is increased. However, when the amount of the ammonia which is close to a saturated state is already adsorbed at the low temperature but the urea aqueous solution is continuously supplied without taking this into consideration, the amount of the ammonia which is in excess of an upper limit corresponding to the adsorption ability is not adsorbed on the catalyst layer, but the ammonia passes through the NOx catalyst as it is. On the other hand, when the urea aqueous solution continues to be supplied at an increasing load operation such as acceleration without taking the amount of the adsorbed ammonia into consideration, the desorption of the adsorbed ammonia occurs owing to the rise of an exhaust temperature, which results in the promotion of the ammonia slip. Therefore, in the urea SCR, it is necessary to supply the urea aqueous solution in consideration of the amount of the ammonia adsorbed on the NOx catalyst (hereinafter referred to as “the ammonia adsorption amount”). Here, concerning the urea SCR in consideration of the ammonia adsorption amount, the following technology is present.
An amount of ammonia to be consumed by the reduction of NOx is calculated from engine NOx emissions and an NOx purification ratio in accordance with a temperature of an NOx catalyst, and a balance obtained by subtracting this calculated amount from the previously calculated amount of held ammonia is calculated as the present amount of the held ammonia (the following patent document 1). Then, an amount of an ammonia solution to be supplied is controlled so that the calculated amount of the held ammonia falls within a predetermined range. Here, a map set beforehand by an experiment or the like is generally used in the calculation of the NOx purification ratio.